How To Replace the Upper and
Lower Thermostat Housing on a
Ford 4.0L V6 SOHC Engine

Ford 4.0L SOHC Engine

More free information by Ben Grosser.

Table of Contents


The Ford 4.0L V6 SOHC engine is found in a number of models, including the Ford Ranger, Ford Sport Trac, Ford Mustang, and the Ford Explorer. It was manufactured starting in 2001 (before that was an OHV version). An increasingly common complaint is a failure of the lower thermostat housing. On my 2003 Ford Ranger with 160k miles, this housing sprung a leak along the seam between its upper and lower portions. When I would push on the throttle, a stream of coolant would squirt out of the housing towards the left side of the engine bay. From there, the radiator fan would blow it all over everything (see photo below). It started out small, but got bigger over time. I finally noticed it when it was bad enough to make me start smelling burnt coolant. By this time, I was loosing about ½ quart of coolant/day and it rapidly got worse.


Unfortunately, this is not a simple gasket swap. To fix this problem you have to replace the lower thermostat housing. You won't find this part at Autozone; you'll have to go to the Ford dealer (all the parts I used are detailed below). It took me about 6 hours to do this job, but with this guide it will likely take you less. This procedure is not detailed in the Haynes manual, nor in the Ford Workshop Manual, so I decided to write it up here.

What follows is a detailed step-by-step guide to performing this swap. A lot of credit goes to 03 Demon 4.0L on He did this swap before me and laid out the essentials. My brother Dan also answered lots of questions. I've added photos and other details so that those with this same problem can do the swap quickly and easily. If you have any questions, or anything to add, please .

Just Want to Replace the Thermostat?

If all you need is to replace the thermostat, then this guide will help you with that too. Start with Step 1 and continue through Step 4, then jump to Step 9.

Tools & Supplies


I suggest you collect all of the tools I list below. For me, none of them were optional. Most importantly, do get the 8 mm flex socket. You can pick one up at Sears for $7.49, and it is money well spent.



Other Tools


Parts List


I ordered these parts from my local Ford dealer. The cost was about $250. You could also order them online from a number of places, but I wanted them right away. I also wanted someone to double check and make sure the parts were correct for my vehicle. The choice is yours.

The parts list below will replace everything from the lower housing gasket on up. You might choose to replace less. For me, as long as I was doing the work, I wanted it all replaced. But the only parts I absolutely had to replace were the gaskets, o-rings, and lower housing. However, some have noted that the temp sensors in their vehicles don't mate up with the latest housing design. Mine looked the same, but I got new ones anyway. Your thermostat may also be working, so perhaps you could reuse that. I went with a new one. I even replaced the bolts because mine were quite rusty. I was happy with my choice, but you might make a different one.

In the last couple years the aftermarket has caught on that this is a common problem and is now manufacturing kits for it. SO apparently you can get the parts at a number of places such as Autozone. However, I have also been told by some that these aftermarket parts are not all of the same quality as the Ford parts (pretty scary since the Ford parts are all failing!).

Ford Part # Motorcraft Part # Part Description
4L5Z-8592-AA RH165 upper thermostat housing
2L5Z-8592-BB RH144 lower thermostat housing
1L2Z-8255-AA RG614 lower thermostat housing seal
-W702837-S300 thermostat seal (aka large o-ring)
2L2Z 8575 AA RT1167 thermostat
3F1Z-10884-AA SW6146 temperature sensor
3L8Z-12A648-BA DY1004 temperature sensor
W706800-S303 retainer clips for sensors (buy 2)
W500015-S437 bolts for upper housing
W702527-S437 bolts for lower housing

If your existing housing only has *1* sensor in it, read this.

Next -> Steps 1-3: Disconnect Battery, Drain Coolant, Remove Hoses